The blood urea nitrogen (BUN) may reflect kidney function but is affected by many other things other than kidney function. The best test to estimate kidney function is the estimated glomerular filtration rate (eGFR). The BUN to creatinine ratio (BUN/Creat) can be elevated in dehydration but is not very reliable. The serum sodium can be low if there is excess water in the blood and can be elevated if there is a deficit of water in the blood. The best way to determine dehydration is to examine the patient first, take a good history of recent events, especially the degree of fluid and nutritional intake, and then examine the laboratory tests to see if they match the clinical impression based on the history and physical examination.